1. Field of the Invention
This invention relates to a lens whose focal length is variable, and more particularly to a variable focal length lens whose focal length can be varied by utilization of the electro-optic effect without any mechanical movement.
2. Description of the Prior Art
Generally, in variable-focal-length optical lens systems called zoom lenses, variation of the focal length thereof is accomplished by moving predetermined lens groups of the lens system along the optical axis thereof and varying the spacing between those lens groups. In these lens systems, however, a moving mechanism for moving the lens groups is necessary and the result is that performances such as rapid change of the focal length, compactness and reduced cost are not sufficiently satisfied, and the advent of lens systems in which those performances have been enhanced has heretofore been desired.
On the other hand, a variable-focal-length lens which is intended to eliminate the above-noted disadvantages by utilization of the electro-optic effect and which is compact and whose focal length can be rapidly changed is proposed e.g. in U.S. Pat. No. 4,466,703. An example of such variable-focal-length lens according to the prior art is shown in FIG. 1 of the accompanying drawings.
In FIG. 1, reference numeral 1 designates KH.sub.2 PO.sub.4 crystal having a linear electro-optic effect (Pockels effect), reference numeral 2 denotes a first transparent electrode portion comprising a plurality of concentric annular transparent electrodes 2.sub.1, 2.sub.2, . . . , 2.sub.n, reference numeral 3 designates a lead wire, reference numeral 4 denotes a planar second transparent electrode portion, reference numeral 5 designates a polarizing plate, and reference numeral 6 denotes a power source unit. The power source unit 6 imparts a potential to each of the annular transparent electrodes 2.sub.1, 2.sub.2, . . . , 2.sub.n through the lead wire 3 and applies an electric field of inclined intensity distribution to the crystal 1, thereby causing the crystal 1 to create a refractive index distribution having a lens action. Also, the focal length of said lens action is varied by varying the electric field to be applied.
However, in the variable-focal-length lens according to the prior art as shown in FIG. 1, means for applying an inclined electric field is necessary to obtain a refractive index distribution having a lens action, and this has led to complicated structure of the lens and the difficulty in manufacturing the lens at low cost. Also, the refractive index distribution thus obtained has been approximately created by the concentric annular electrodes and the reduced performance of the lens such as aberrations has been unavoidable.